Really haven't thought of a gametype, but I would make a one bomb variation of Forge World.

Attackers start on the Island.

Defenders spawn either the little nook or where Blood Gulch is.

Attackers have to get to the base via 2 Falcons, along the way there will be 4 AA guns(The Purple Covenant Gun Platforms), in the defenders base there will be 4 teleporters, one leading to each gun. When you go through a teleporter you will wind up in a small area with a one-sided shield door and a teleporter(Just in case the gun you went to is taken). Once you go through the door you have die to get back. The Falcons will have a guided path to the base, but will have to destroy the guns along the way.

May just put the Defenders base in the air. The Defenders will have 2 banshees, but no Laser since a person going through the teleporter with a laser would cause major problems.

The base would be pretty big, and the teams will have to jump off so the Falcon Pilots can give them cover.

8 vs 8 is recommended.

May even add a blocked teleporter and if the Attackers can destroy the object blocking it. The teleporter on the island will open.

Obviously not exactly like that but rather a Forerunner style based off that idea. Multiple levels that you work you way up to plant the bomb/capture the objective. Or in Slayer perhaps a Falcon to use against the other team.